I've been calculating my mpg for several years with my daily driven stock '90 245 AW70 which includes a variety of city and highway driving. I actually just put 900 miles on it this past weekend between my gf's parents house in the Poconos, back to our place in Baltimore, as well as my Parents house outside of Philadelphia. I average about 19 city and 24 hwy, a bit better coming back from the Poconos because its all down hill. I typically drive between 75/80mph on the highway turning ~3k rpm, I'm really not shooting for good mileage, I just monitor it.
These numbers have been very steady over the past few years, but I plan on pulling the intake mani, cleaning it out, putting in lower mile injectors and replacing the FPR and intake gasket; hopefully I can pull a little more out of it.

Your numbers are pretty low, I would definitely look into what issues might be occurring. Have you ever cleaned your throttle body? Tune up? What do your plugs look like?

Have you checked for any stored codes in the OBD?